This invention relates generally to an implantable cardiac stimulation device capable of sensing cardiac signals in at least one atrial chamber of the heart and one ventricular chamber of the heart. More specifically, the present invention is directed to a method for automatically determining a pre-ventricular atrial blanking period in order to prevent sensing of far-field R-waves in the atria.
In a normal human heart, the sinus node, generally located near the junction of the superior vena cava and the right atrium, constitutes the primary natural pacemaker initiating rhythmic electrical excitation of the heart chambers. The cardiac impulse arising from the sinus node is transmitted to the two atrial chambers, causing a depolarization known as a P-wave and the resulting atrial chamber contractions. The excitation pulse is further transmitted to and through the ventricles via the atrioventricular (A-V) node and a ventricular conduction system causing a depolarization known as an R-wave and the resulting ventricular chamber contractions.
Disruption of this natural pacemaking and conduction system as a result of aging or disease can be successfully treated by artificial cardiac pacing using implantable cardiac stimulation devices, including pacemakers and implantable defibrillators, which deliver rhythmic electrical pulses or other anti-arrhythmia therapies to the heart, via electrodes implanted in contact with the heart tissue, at a desired energy and rate. One or more heart chambers may be electrically stimulated depending on the location and severity of the conduction disorder.
A single-chamber pacemaker delivers pacing pulses to one chamber of the heart, either one atrium or one ventricle. Dual chamber pacemakers are now commonly available and can provide stimulation in both an atrial chamber and a ventricular chamber, typically the right atrium and the right ventricle. Both unipolar or bipolar dual chamber pacemakers exist in which a unipolar or bipolar lead extends from an atrial channel of the dual chamber device to the desired atrium (e.g. the right atrium), and a separate unipolar or bipolar lead extends from a ventricular channel to the corresponding ventricle (e.g. the right ventricle). In dual chamber, demand-type pacemakers, commonly referred to as DDD pacemakers, each atrial and ventricular channel includes a sense amplifier to detect cardiac activity in the respective chamber and an output circuit for delivering stimulation pulses to the respective chamber.
If an intrinsic atrial depolarization signal (a P-wave) is not detected by the atrial channel, a stimulating pulse will be delivered to depolarize the atrium and cause contraction. Following either a detected P-wave or an atrial pacing pulse, the ventricular channel attempts to detect a depolarization signal in the ventricle, known as an R-wave. If no R-wave is detected within a defined atrial-ventricular interval (AV interval, also referred to as AV delay), a stimulation pulse is delivered to the ventricle to cause ventricular contraction. In this way, rhythmic dual chamber pacing is achieved by coordinating the delivery of ventricular output in response to a sensed or paced atrial event.
One problem faced with the advent of dual-chambered pacemakers is that when a pacemaker delivers a stimulation pulse to the ventricle during an appropriate portion of a cardiac cycle, this pulse would be sensed on the atrial channel. Therefore, it has been a common practice to apply a post-ventricular atrial blanking (PVAB) period upon delivery of a ventricular stimulation pulse. This practice prevents saturation of the sense amplifiers of the atrial channel during the delivery of the ventricular pulse.
By disabling the atrial sense amplifier upon the delivery of a ventricular stimulating pulse, the atrial sense amplifier is not affected by the ventricular stimulation pulse. At a specified time interval after the delivery of a ventricular stimulating pulse, the atrial sense amplifiers are enabled again to sense intrinsic or evoked atrial events. The post ventricular atrial blanking period is typically on the order of 150 msec.
The post-ventricular atrial blanking period poses a new problem in that it may occur mid-way or even late in the atrial cycle, and may therefore result in the atrial channel not sensing the next intrinsic atrial event. Essentially, the atrial channel is xe2x80x9cblindedxe2x80x9d to rapid atrial rates, precluding proper diagnostic and therapeutic measures to be taken by the implanted cardiac device.
Instead, a missed atrial event would trigger an atrial stimulation pulse to be inappropriately delivered by the stimulation device. Such inappropriate pacing could endanger the patient by inducing a sequence of events that might induce cardiac arrhythmias. Therefore, the post-ventricular atrial blanking period is preferably kept as short as possible to allow sensing of high atrial rates, but long enough to prevent detection of the ventricular stimulation pulse or the subsequent afterpotential.
However, two other events may follow an intrinsic ventricular event that may also be sensed by the atrial channel: 1) a far-field R-wave due to the depolarization of the large ventricular mass creating a signal large enough to be detected in the atria, and 2) a retrograde conducted depolarization in patients with conduction pathways that allow the ventricular R-wave to be conducted back into the atria. Either of these events may be incorrectly detected by the atrial channel as an intrinsic P-wave, resulting in the false interpretation of the atrial rhythm.
Such atrial detection would trigger the delivery of another ventricular stimulation pulse at the end of a PV interval. This situation could lead to pacemaker-mediated tachycardia (PMT) since the ventricular stimulation rate is tracking a falsely interpreted atrial rate. To address this problem, the post-ventricular atrial blanking period is typically followed by a post-ventricular atrial refractory period (PVARP). The post-ventricular atrial refractory period is typically 100 to 150 msec long and allows detection of events, such as far-field R-waves or retrograde P-waves, but any events detected during this refractory period are not tracked for the purposes of delivering ventricular stimulation.
A far-field R-wave may also be detected by the atrial sensing circuits prior to the detection of an R-wave by the ventricular channel. In this situation, a post-ventricular atrial refractory period is not effective in preventing tracking of the inappropriately detected far-field R-wave by the atrial channel. Depending on electrode position, atrial sensitivity settings, and other factors, the likelihood of sensing far-field R-waves on the atrial channel will vary between patients. Because an intrinsic R-wave is not always sensed instantaneously, it is possible in some patients that a far-field R-wave is sensed on the atrial channel prior to the source R-wave being detected on the ventricular channel.
A far-field ventricular event inappropriately sensed by the atrial channel as a P-wave could cause a ventricular stimulation pulse to be delivered to the ventricle at a time when the ventricle has already depolarized. Stimulation of the ventricle during a time when the ventricle cannot depolarize will result in a loss of capture detection in devices equipped with automatic capture verification and thus invoke a high-energy back-up stimulation pulse and possibly a threshold test when these responses are not clinically indicated. In a worst-case scenario, stimulation of the ventricle during its repolarization phase can induce ventricular fibrillation, a potentially lethal result. Sensing a far-field ventricular signal by the atrial channel may also trigger automatic mode-switching due to the interpretation of a high atrial rate.
It would be desirable therefore to provide an automatically adjustable atrial blanking period in dual chamber or multi-chamber stimulation devices in order to ensure effective elimination of far-field R-wave sensing despite individual variations that affect the likelihood of far-field R-wave sensing.
The present invention addresses this need by providing an implantable cardiac stimulation device capable of dynamically adjusting a pre-ventricular atrial blanking period for the prevention of far-field R-wave sensing.
When operating according to an illustrative embodiment, the control system determines a minimum interval between an atrial event, either an atrial stimulation pulse or an intrinsic P-wave, and the subsequent intrinsic R-wave, known as the AR or PR interval, respectively. A minimum atrial refractory period is determined next by determining the shortest atrial refractory period setting that prevents two consecutive atrial events from being sensed with no intervening ventricular event. The maximum pre-ventricular atrial blanking period is then calculated as the difference between the minimum (PR or AR) interval and the minimum atrial refractory period.
Next, the pre-ventricular atrial blanking period is optimized to allow a maximum window of time for sensing atrial events by progressively shortening the pre-ventricular atrial blanking period from the calculated maximum until two consecutive atrial events occur, the second event presumably being a far-field R-wave. The pre-ventricular atrial blanking period is then set to a value equal to the setting at which far-field R-wave sensing first occurred plus a predefined safety margin.
The methods of the present invention thus provide automatic optimization of a pre-ventricular atrial blanking period for reliably preventing far-field R-wave sensing on the atrial channel of a dual-chamber or multi-chamber cardiac stimulation device. The algorithm presented herein may be repeated periodically such that the pre-ventricular atrial blanking period may be adjusted in response to varying conditions that increase or decrease the likelihood of far-field R-wave sensing. Stimulation device performance is improved by preventing inappropriate device responses such as mode-switching, or ventricular tracking, due to atrial detection of far-field R-waves as P-waves.